The prevalence of childhood obesity in the United States remains at historically high levels despite evidence of a recent plateau in some communities. Evidence shows that even infants experienced a dramatic rise in excess weight in the past 20 years, implying that the roots of the epidemic can be found as early as infancy. Trends in insufficient sleep have paralleled the increasing trends in childhood obesity. Across infancy, childhood, and adolescence, sleep duration has decreased by 30 to 60 minutes over the last 20 years, with the most pronounced decreases in sleep among children under 3 years of age. Mounting epidemiologic evidence indicates that short duration of sleep is a risk factor for obesity. We and others have observed an inverse association between sleep duration and obesity in longitudinal studies of children, even as young as infancy. Yet, major questions remain regarding the extent to which patterns of sleep other than short sleep duration predict infant growth and energy balance including sleep consolidation/fragmentation, day-to-day variability in sleep patterns, and sleep timing. Prior studies in children are also limited by a reliance on parent report of sleep rather than use of validated, objective measures such as actigraphy. Furthermore, few studies have tested mechanisms linking sleep and growth in infants. Adverse sleep patterns may promote accelerated infant weight gain through effects on feeding behaviors and neurobehavioral pathways including decreased self- regulation of eating, behavior, and emotion. Finally, there are gaps in our understanding of contextual determinants of infant sleep which could inform intervention development. The goals of this study are three- fold. First, we will examine modifiable determinants of infant sleep patterns that will directly inform interventions to improve sleep and growth patterns in the first 24 months of life. Second, we will examine associations of infant sleep patterns with growth from birth to 24 months of age. Third, we will explore feeding and neurobehavioral pathways linking adverse sleep patterns to early childhood obesity. We will carry out this project within a cohort of 400 infants, and their caregivers, that will be recruited from a large newborn nursery in Massachusetts General Hospital (MGH) and leverage the existing electronic health records from their pediatric primary care visits across the MGH health care system. This study will focus on an ethnically and socio-economically diverse sample of newborns and their families, and use parent report, in home observations, and actigraphic estimates of sleep. The results of this study could strengthen our understanding of the role of sleep patterns on infant growth trajectories in early life. Furthermore, our study will supply key data needed for the design of future intervention studies, such as examining sleep patterns that confer increased risk of obesity, identifying important proximate behaviors that mediate these associations, and identifying socio-cultural/environmental factors as potential targets for sleep improvement interventions.